Flexible coupling for geared turbine engine

ABSTRACT

A gas turbine engine includes a fan, a fan shaft coupled with the fan and arranged along an engine central axis, and a frame supporting the fan shaft. The frame defines a lateral frame stiffness (LFS). A non-rotatable flexible coupling and a rotatable flexible coupling support an epicyclic gear system. The couplings are subject to a Motion II of cantilever beam free end motion with respect to the engine central axis. The non-rotatable and the rotatable flexible couplings each have a stiffness of a common stiffness type under a common type of motion. The common stiffness type is a Stiffness B and the common type of motion is the Motion II. The Stiffness B of the rotatable flexible coupling is greater than the stiffness B of the non-rotatable flexible coupling, and a ratio of LFS/Stiffness B of the non-rotatable flexible coupling is in a range of 10-40.

CROSS REFERENCE TO RELATED APPLICATIONS

The present disclosure is a continuation of U.S. patent application Ser.No. 17/034,713 filed Sep. 28, 2020, which is continuation of U.S. patentapplication Ser. No. 16/148,239, filed Oct. 1, 2018, which is acontinuation of U.S. patent application Ser. No. 15/862,716, filed Jan.5, 2018, which is a continuation of U.S. patent application Ser. No.14/766,766, filed Aug. 10, 2015, now U.S. Pat. No. 9,863,326 grantedJan. 9, 2018, which is a national application of InternationalApplication No. PCT/US2014/016753, filed Feb. 18, 2014, which claimsbenefit of U.S. Provisional Application No. 61/777,320 filed Mar. 12,2013.

BACKGROUND

A gas turbine engine typically includes a fan section, a compressorsection, a combustor section and a turbine section. Air entering thecompressor section is compressed and delivered into the combustionsection where it is mixed with fuel and ignited to generate a high-speedexhaust gas flow. The high-speed exhaust gas flow expands through theturbine section to drive the compressor and the fan section. Thecompressor section typically includes low and high pressure compressors,and the turbine section typically includes low and high pressureturbines.

The high pressure turbine drives the high pressure compressor through anouter shaft to form a high spool, and the low pressure turbine drivesthe low pressure compressor through an inner shaft to form a low spool.The fan section may also be driven by the low inner shaft. A directdrive gas turbine engine includes a fan section driven by the low spoolsuch that the low pressure compressor, low pressure turbine and fansection rotate at a common speed in a common direction.

A speed reduction device such as an epicyclical gear assembly may beutilized to drive the fan section such that the fan section may rotateat a speed different than the turbine section. In such enginearchitectures, a shaft driven by one of the turbine sections provides aninput to the epicyclical gear assembly that drives the fan section at areduced speed. During flight, a geared engine can be subject to aero andmaneuver loads that cause significant engine deflections. The loads cancause different types of deflection motions, as will be described inmore detail below, between a gear system and static portions of theengine such that the gear system can have the tendency to misalign withrespect to the engine central axis. Misalignment of the gear system cancause efficiency losses in the meshing between gear teeth in the gearsystem and reduced life from increases in concentrated stresses.

SUMMARY

A gas turbine engine according to an example of the present disclosureincludes a fan, a fan shaft coupled with the fan and arranged along anengine central axis, and a frame supporting the fan shaft. The framedefines a lateral frame stiffness (LFS). An epicyclic gear system iscoupled to the fan shaft, and a non-rotatable flexible coupling and arotatable flexible coupling support the epicyclic gear system. Thenon-rotatable flexible coupling and the rotatable flexible coupling aresubject to a Motion II of cantilever beam free end motion with respectto the engine central axis. The non-rotatable flexible coupling and therotatable flexible coupling each have a stiffness of a common stiffnesstype under a common type of motion with respect to the engine centralaxis. The common stiffness is defined with respect to the LFS. Thecommon stiffness type is a Stiffness B and the common type of motion isthe Motion II. The Stiffness B of the rotatable flexible coupling isgreater than the stiffness of the non-rotatable flexible coupling, and aratio of LFS/Stiffness B of the non-rotatable flexible coupling is in arange of 10-40.

In a further embodiment of any of the foregoing embodiments, a ratioLFS/Stiffness B of the rotatable flexible coupling is in a range of33-1000.

In a further embodiment of any of the foregoing embodiments, the commontype of motion is selected from Motion I, Motion III, or Motion IV,where Motion I is parallel offset guided end motion, Motion III isangular misalignment no offset motion, and Motion IV is axial motion.

In a further embodiment of any of the foregoing embodiments, theepicyclic gear system includes a sun gear in meshed engagement withmultiple intermediate gears that are rotatably mounted on bearings in arotatable carrier. Each intermediate gear is in meshed engagement with anon-rotatable ring gear. The sun gear is rotatably coupled to the fanshaft, and the first, non-rotatable flexible coupling is coupled withthe non-rotatable ring gear.

In a further embodiment of any of the foregoing embodiments, the commonstiffness type is Stiffness A under Motion IV, and a ratio ofLFS/Stiffness A of the non-rotatable flexible coupling is in a range of6-25, and a ratio of LFS/Stiffness A of the rotatable flexible couplingis in a range of 28-200.

In a further embodiment of any of the foregoing embodiments, the commonstiffness type is Stiffness C under Motion I, and a ratio ofLFS/Stiffness C of the non-rotatable flexible coupling is in a range of1.5-7, and a ratio LFS/Stiffness C of the rotatable flexible coupling isin a range of 16-100.

In a further embodiment of any of the foregoing embodiments, the commonstiffness type is Stiffness D under Motion I, and a ratio ofLFS/Stiffness D of the non-rotatable flexible coupling is in a range of0.25-0.5, and a ratio LFS/Stiffness D of the rotatable flexible couplingis in a range of 2-100.

In a further embodiment of any of the foregoing embodiments, the commonstiffness type is Stiffness E under Motion III, and a ratio ofLFS/Stiffness E of the non-rotatable flexible coupling is in a range of6-40, and a ratio LFS/Stiffness E of the rotatable flexible coupling isin a range of 4-500.

In a further embodiment of any of the foregoing embodiments, theepicyclic gear system is coupled through an input shaft to a lowpressure turbine, the low pressure turbine having a pressure ratio ofgreater than 5.s

A gas turbine engine according to an example of the present disclosureincludes a fan, a fan shaft coupled with the fan and arranged along anengine central axis, and a frame supporting the fan shaft. The framedefines a lateral frame stiffness (LFS). An epicyclic gear system iscoupled to the fan shaft, and a non-rotatable flexible coupling and arotatable flexible coupling support the epicyclic gear system. Thenon-rotatable flexible coupling and the rotatable flexible coupling aresubject to a Motion I of parallel offset guided end motion with respectto the engine central axis. The non-rotatable flexible coupling and therotatable flexible coupling each have a stiffness of a common stiffnesstype under a common type of motion with respect to the engine centralaxis. The stiffness is defined with respect to the LFS, the commonstiffness type is a Stiffness C and the common type of motion is theMotion I. The Stiffness C of the rotatable flexible coupling is greaterthan the stiffness of the non-rotatable flexible coupling, and a ratioof LFS/Stiffness C of the non-rotatable flexible coupling is in a rangeof 1.5-7.

In a further embodiment of any of the foregoing embodiments, a ratioLFS/Stiffness C of the rotatable flexible coupling is in a range of16-100.

In a further embodiment of any of the foregoing embodiments, the commontype of motion is selected from Motion I, Motion III, or Motion IV,where Motion I is parallel offset guided end motion, Motion III isangular misalignment no offset motion, and Motion IV is axial motion,and wherein the epicyclic gear system includes a sun gear in meshedengagement with multiple intermediate gears that are rotatably mountedon bearings in a rotatable carrier. Each intermediate gear is in meshedengagement with a non-rotatable ring gear. The sun gear is rotatablycoupled to the fan shaft, and the first, non-rotatable flexible couplingis coupled with the non-rotatable ring gear.

In a further embodiment of any of the foregoing embodiments, the commonstiffness type is Stiffness A under Motion IV, and a ratio ofLFS/Stiffness A of the non-rotatable flexible coupling is in a range of6-25, and a ratio of LFS/Stiffness A of the rotatable flexible couplingis in a range of 28-200.

In a further embodiment of any of the foregoing embodiments, the commonstiffness type is Stiffness D under Motion I, and a ratio ofLFS/Stiffness D of the non-rotatable flexible coupling is in a range of0.25-0.5, and a ratio LFS/Stiffness D of the rotatable flexible couplingis in a range of 2-100.

In a further embodiment of any of the foregoing embodiments, the commonstiffness type is Stiffness E under Motion III, and a ratio ofLFS/Stiffness E of the non-rotatable flexible coupling is in a range of6-40, and a ratio LFS/Stiffness E of the rotatable flexible coupling isin a range of 4-500.

A gas turbine engine according to an example of the present disclosureincludes a fan, a fan shaft coupled with the fan and arranged along anengine central axis, and a frame supporting the fan shaft. The framedefines a lateral frame stiffness (LFS). An epicyclic gear system iscoupled to the fan shaft, and a non-rotatable flexible coupling and arotatable flexible coupling support the epicyclic gear system. Thenon-rotatable flexible coupling and the rotatable flexible coupling aresubject to a Motion III of angular misalignment no offset motion withrespect to the engine central axis. The non-rotatable flexible couplingand the rotatable flexible coupling each have a stiffness of a commonstiffness type under a common type of motion with respect to the enginecentral axis. The stiffness is defined with respect to the LFS. Thecommon stiffness type is a Stiffness E and the common type of motion isthe Motion III. The Stiffness E is defined with respect to the LFS, theStiffness E of the rotatable flexible coupling are greater than thestiffness of the non-rotatable flexible coupling, and a ratio ofLFS/Stiffness E of the non-rotatable flexible coupling is in a range of6-40.

In a further embodiment of any of the foregoing embodiments, a ratio ofLFS/Stiffness E of the rotatable flexible coupling is in a range of4-500.

In a further embodiment of any of the foregoing embodiments, the commontype of motion is selected from Motion I, Motion III, or Motion IV,where Motion I is parallel offset guided end motion, Motion III isangular misalignment no offset motion, and Motion IV is axial motion,and wherein the epicyclic gear system includes a sun gear in meshedengagement with multiple intermediate gears that are rotatably mountedon bearings in a rotatable carrier. Each intermediate gear is in meshedengagement with a non-rotatable ring gear. The sun gear is rotatablycoupled to the fan shaft, and the first, non-rotatable flexible couplingis coupled with the non-rotatable ring gear.

In a further embodiment of any of the foregoing embodiments, the commonstiffness type is Stiffness A under Motion IV, and a ratio ofLFS/Stiffness A of the non-rotatable flexible coupling is in a range of6-25, and a ratio of LFS/Stiffness A of the rotatable flexible couplingis in a range of 28-200.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example gas turbine engine.

FIG. 2 illustrates selected portions of the engine of FIG. 1 .

FIG. 3 schematically illustrates parallel offset guided end motion of aflexible coupling in the engine of FIG. 1 .

FIG. 4 schematically illustrates cantilever beam free end motion of aflexible coupling in the engine of FIG. 1 .

FIG. 5 schematically illustrates angular misalignment no offset motionof a flexible coupling in the engine of FIG. 1 .

FIG. 6 schematically illustrates axial motion of a flexible coupling inthe engine of FIG. 1 .

FIG. 7 schematically illustrates torsional motion of a flexible couplingin the engine of FIG. 1 .

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. Alternative engines mightinclude an augmentor section (not shown) among other systems orfeatures. The fan section 22 drives air along a bypass flow path B in abypass duct defined within a nacelle 15, while the compressor section 24drives air along a core flow path C for compression and communicationinto the combustor section 26 then expansion through the turbine section28. Although depicted as a two-spool turbofan gas turbine engine in thedisclosed non-limiting embodiment, it should be understood that theconcepts described herein are not limited to use with two-spoolturbofans as the teachings may be applied to other types of turbineengines, including three-spool architectures.

The engine 20 includes a low speed spool 30 and a high speed spool 32mounted for rotation about an engine central axis A relative to anengine static structure 36 via several bearing systems, shown at 38,38B, 38C and 38D. It is to be understood that various bearing systems atvarious locations may alternatively or additionally be provided, and thelocation of bearing systems may be varied as appropriate to theapplication.

The low speed spool 30 includes an inner shaft 40 that interconnects afan 42, a low pressure compressor 44 and a low pressure turbine 46. Theinner shaft 40 is connected to the fan 42 through a speed changemechanism, which in this example is a gear system 48, to drive the fan42 at a lower speed than the low speed spool 30. The high speed spool 32includes an outer shaft 50 that interconnects a high pressure compressor52 and high pressure turbine 54. A combustor 56 is arranged between thehigh pressure compressor 52 and the high pressure turbine 54. Amid-turbine frame 57 of the engine static structure 36 is arrangedgenerally between the high pressure turbine 54 and the low pressureturbine 46. The mid-turbine frame 57 further supports bearing 38D in theturbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via, for example, bearing systems 38C and 38Dabout the engine central axis A which is collinear with theirlongitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and gear system 48 can be varied. For example, gear system48 may be located aft of combustor section 26 or even aft of turbinesection 28, and fan section 22 may be positioned forward or aft of thelocation of gear system 48.

The engine 20 in one example is a high-bypass geared engine. In afurther example, the engine 20 has a bypass ratio that is greater thanabout six (6), with an example embodiment being greater than about ten(10), the gear system 48 is an epicyclic gear train, such as a planet orstar gear system, with a gear reduction ratio of greater than about 2.3and the low pressure turbine 46 has a pressure ratio that is greaterthan about five. In one disclosed embodiment, the bypass ratio isgreater than about ten (10:1), the fan diameter is significantly largerthan that of the low pressure compressor 44, and the low pressureturbine 46 has a pressure ratio that is greater than about five 5:1. Lowpressure turbine 46 pressure ratio is pressure measured prior to inletof low pressure turbine 46 as related to the pressure at the outlet ofthe low pressure turbine 46 prior to an exhaust nozzle. The gear system48 can be an epicycle gear train, such as a planet or star gear system,with a gear reduction ratio of greater than about 2.3:1. It is to beunderstood, however, that the above parameters are only exemplary andthat the present disclosure is applicable to other gas turbine engines.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, withthe engine at its best fuel consumption—also known as “bucket cruiseThrust Specific Fuel Consumption (‘TSFC’)”—is the industry standardparameter of lbm of fuel being burned divided by lbf of thrust theengine produces at that minimum point. “Low fan pressure ratio” is thepressure ratio across the fan blade alone, without a Fan Exit Guide Vane(“FEGV”) system. The low fan pressure ratio as disclosed hereinaccording to one non-limiting embodiment is less than about 1.45. “Lowcorrected fan tip speed” is the actual fan tip speed in ft/sec dividedby an industry standard temperature correction of [(Tram°R)/(518.7°R)]^(0.5). The “Low corrected fan tip speed” as disclosedherein according to one non-limiting embodiment is less than about 1150ft/second.

As described below, the gear system 48 in the engine 20 is mounted onflexible couplings 74 (FIG. 2 ) to reduce loads on the gear system 48due to misalignment with respect to the engine central axis A. As aresult, the embodiments hereafter described resolve the aforementionedissues associated with respect to misalignment in the gear system thatwould otherwise result in efficiency losses in the gear teeth in thegear system and reduced life from increases in concentrated stresses.

FIG. 2 schematically shows a portion of the engine 20 around the gearsystem 48. The gear system 48 is driven by the low speed spool 30through an input shaft 60. The input shaft 60 transfers torque to thegear system 48 from the low speed spool 30. In this example, the inputshaft 60 is coupled to a sun gear 62 of the gear system 48. The sun gear62 is in meshed engagement with multiple intermediate gears 64, of whichthe illustrated intermediate gear 64 is representative. Eachintermediate gear 64 is rotatably mounted in a carrier 66 by arespective rolling bearing 68, such as a journal bearing. Rotary motionof the sun gear 62 urges each intermediate gear 64 to rotate about arespective longitudinal axis P.

Each intermediate gear 64 is also in meshed engagement with a ring gear70 that is rotatably coupled to a fan shaft 72 in this example. Sincethe intermediate gears 64 mesh with the rotating ring gear 70 and therotating sun gear 62, the intermediate gears 64 rotate about their ownaxes to drive the ring gear 70 to rotate about engine central axis A.The rotation of the ring gear 70 is conveyed to the fan 42 through thefan shaft 72 to thereby drive the fan 42 at a lower speed than the lowspeed spool 30. In this example, the carrier 66 is fixed (non-rotating)and the ring gear 70 is rotatable such that the intermediate gears 64serve as star gears. In any of the examples herein, the carrier 66 canalternatively be rotatable and the ring gear 70 can be fixed(non-rotating) such that the intermediate gears 64 serve as planet gearsand the carrier is coupled to rotatably drive the fan shaft 72 and thefan 42. Thus, the flexible support 76 described herein can be coupledeither to the fixed carrier (star system) or to the fixed ring gear(planetary system), depending upon the configuration of the gear system48.

The gear system 48 is at least partially supported by flexible couplings74. In FIG. 2 , the flexible couplings 74 include a first flexiblecoupling, which is flexible support 76 that is coupled with the carrier66 and a second flexible coupling, which is the input shaft 60 thatsupports the gear system 48 with respect to bearing system 38C. Theflexible support 76 is static (fixed, non-rotating) and supports thegear system 48 with respect to the static structure 36.

The static structure 36 includes a bearing support static structure 78,which can also be termed a “K-frame.” In this example, the bearingsupport static structure 78 is the support structure forward of the gearsystem 48 that supports the bearings 38A and 38B and the fan shaft 72.The bearing support static structure 78 defines a lateral framestiffness, represented as “LFS” in FIG. 2 . The lateral frame stiffnessLFS serves as a reference stiffness from which the different types ofstiffnesses, described below, of the flexible couplings 74 are defined.The term “lateral” or variations thereof as used herein refers to aperpendicular direction with respect to the engine central axis A. It isfurther to be understood that “stiffness” as used herein canalternatively be termed “spring rate.” The stiffnesses, or spring rates,are in units of pounds per inch, although conversions can be used torepresent the units of pounds per inch in other units.

The flexible couplings 74 each have one or more specific stiffnesses A,B, C, D and E, generally represented in FIG. 2 at S1 and S2. Each of thespecific stiffnesses A, B, C, D and E are defined with respect to thelateral frame stiffness LFS and a different type of motion that theflexible couplings 74 can be subject to with respect to the enginecentral axis A. For example, as summarized in Table 1 below, the typesof motion include Motion I, Motion II, Motion III, Motion IV, orcombinations thereof, where Motion I is parallel offset guided endmotion, Motion II is cantilever beam free end motion and Motion III isangular misalignment no offset motion and Motion IV is axial motion.Stiffness A is axial stiffness under Motion IV, Stiffness B is radialstiffness under Motion II, Stiffness C is radial stiffness under MotionI, Stiffness D is torsional stiffness under Motion I, and Stiffness E isangular stiffness under Motion III. Terms such as “radial,” “axial,”“forward” and the like are relative to the engine central axis A.

Motion I, Motion II, Motion III, Motion IV are schematically shown inforce coupling diagrams in, respectively, FIG. 3 , FIG. 4 , FIG. 5 andFIG. 6 , where F represents an applied load or force and M represents aresulting moment of force. An applied force can also result in torsionalmotion, as represented in FIG. 7 , as well as lateral motion. The term“torsion” or variations thereof as used herein refers to a twistingmotion with respect to the engine central axis A. In this regard, one orboth of the flexible couplings 74 also has a torsional stiffness TS anda lateral stiffness LS defined with respect to the lateral framestiffness LFS.

TABLE 1 Types of Motion Type of Motion Description I parallel offsetguided end motion II cantilever beam free end motion III angularmisalignment no offset motion IV axial motion

In one example, the torsional stiffness TS and the lateral stiffness LSof one or both of the flexible couplings 74 are selected in accordancewith one another to reduce loads on the gear system 48 from misalignmentof the gear system 48 with respect to the engine central axis A. Thatis, the torsional stiffness TS and the lateral stiffness LS of theflexible support 76 can be selected in accordance with one another, andthe torsional stiffness TS and the lateral stiffness LS of the inputshaft 60 can be selected in accordance with one another.

For example, a ratio of TS/LS is greater than or equal to about 2 forthe flexible support 76, the input shaft 60 or both individually. Theratio of greater than or equal to about 2 provides the flexiblecouplings 74 with a high torsional stiffness relative to lateralstiffness such that the flexible coupling 74 is permitted to deflect orfloat laterally with relatively little torsional wind-up. Thenomenclature of a ratio represented as value 1/value 2 represents value1 divided by value 2, although the ratios herein can also beequivalently represented by other nomenclatures. As an example, theratio can also be equivalently represented as 2:1 or 2/1. Thestiffnesses herein may be provided in units of pounds per inch, althoughthe ratios herein would be equivalent for other units.

The stiffnesses A, B, C, D, E, TS and LS can also be utilizedindividually or in any combination to facilitate the segregation of thegear system 48 from vibrations and other transients to reduce loads onthe gear system 48 from misalignment of the gear system 48 with respectto the engine central axis A. The following examples, further illustrateselected stiffnesses A, B, C, D, E defined with respect to the framelateral stiffness LFS.

In one example, a ratio of LFS/Stiffness A of the flexible support 76 isin a range of 6-25, and a ratio of LFS/Stiffness A of the input shaft 60is in a range of 28-200.

In another example, a ratio of LFS/Stiffness B of flexible support 76 isin a range of 10-40, and a ratio LFS/Stiffness B of the input shaft 60is in a range of 33-1000.

In another example, a ratio of LFS/Stiffness C of the flexible support76 is in a range of 1.5-7, and a ratio LFS/Stiffness C of the inputshaft 60 is in a range of 16-100.

In another example, a ratio of LFS/Stiffness D of the flexible support76 is in a range of 0.25-0.5, and a ratio LFS/Stiffness D of the inputshaft 60 is in a range of 2-100.

In another example, a ratio of LFS/Stiffness E of the flexible support76 is in a range of 6-40, and a ratio LFS/Stiffness E of the input shaft60 is in a range of 4-500.

In another example, one or more of Stiffness A, Stiffness B, Stiffness Cand Stiffness D of the flexible support 76 is greater than,respectively, Stiffness A, Stiffness B, Stiffness C and Stiffness D ofthe input shaft 60.

In a further example, the flexible support 76 and the input shaft 60have any combination of some or all of the above-described ratios. Theratios are summarized in Table 2 below.

TABLE 2 Ratio Ranges for First and Second Couplings Ratio FLS/StiffnessType of Stiffness Type of Motion flexible support 76 input shaft 60 A IV 6-25  28-200 B II 10-40  33-1000 C I 1.5-7    16-100 D I 0.25-0.5  2-100 E III  6-40  4-500

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

What is claimed is:
 1. A gas turbine engine comprising: a fan; a fanshaft coupled with the fan and arranged along an engine central axis; aframe supporting the fan shaft, the frame defining a lateral framestiffness (LFS); an epicyclic gear system coupled to the fan shaft; anda non-rotatable flexible coupling and a rotatable flexible couplingsupporting the epicyclic gear system, the non-rotatable flexiblecoupling and the rotatable flexible coupling being subject to a MotionII of cantilever beam free end motion with respect to the engine centralaxis, the non-rotatable flexible coupling and the rotatable flexiblecoupling each having a stiffness of a common stiffness type under acommon type of motion with respect to the engine central axis, thecommon stiffness being defined with respect to the LFS, the commonstiffness type is a Stiffness B and the common type of motion is theMotion II, the Stiffness B of the rotatable flexible coupling beinggreater than the stiffness of the non-rotatable flexible coupling, and aratio of LFS/Stiffness B of the non-rotatable flexible coupling is in arange of 10-40.
 2. The gas turbine engine as recited in claim 1, whereina ratio LFS/Stiffness B of the rotatable flexible coupling is in a rangeof 33-1000.
 3. The gas turbine engine as recited in claim 2, wherein thecommon type of motion is selected from Motion I, Motion III, or MotionIV, where Motion I is parallel offset guided end motion, Motion III isangular misalignment no offset motion, and Motion IV is axial motion. 4.The gas turbine engine as recited in claim 3, wherein the epicyclic gearsystem includes a sun gear in meshed engagement with multipleintermediate gears that are rotatably mounted on bearings in a rotatablecarrier, each intermediate gear is in meshed engagement with anon-rotatable ring gear, the sun gear is rotatably coupled to the fanshaft, and the first, non-rotatable flexible coupling is coupled withthe non-rotatable ring gear.
 5. The gas turbine engine as recited inclaim 4, wherein the common stiffness type is Stiffness A under MotionIV, and a ratio of LFS/Stiffness A of the non-rotatable flexiblecoupling is in a range of 6-25, and a ratio of LFS/Stiffness A of therotatable flexible coupling is in a range of 28-200.
 6. The gas turbineengine as recited in claim 5, wherein the common stiffness type isStiffness C under Motion I, and a ratio of LFS/Stiffness C of thenon-rotatable flexible coupling is in a range of 1.5-7, and a ratioLFS/Stiffness C of the rotatable flexible coupling is in a range of16-100.
 7. The gas turbine engine as recited in claim 6, wherein thecommon stiffness type is Stiffness D under Motion I, and a ratio ofLFS/Stiffness D of the non-rotatable flexible coupling is in a range of0.25-0.5, and a ratio LFS/Stiffness D of the rotatable flexible couplingis in a range of 2-100.
 8. The gas turbine engine as recited in claim 7,wherein the common stiffness type is Stiffness E under Motion III, and aratio of LFS/Stiffness E of the non-rotatable flexible coupling is in arange of 6-40, and a ratio LFS/Stiffness E of the rotatable flexiblecoupling is in a range of 4-500.
 9. The gas turbine engine as recited inclaim 8, wherein the epicyclic gear system is coupled through an inputshaft to a low pressure turbine, the low pressure turbine having apressure ratio of greater than
 5. 10. A gas turbine engine comprising: afan; a fan shaft coupled with the fan and arranged along an enginecentral axis; a frame supporting the fan shaft, the frame defining alateral frame stiffness (LFS); an epicyclic gear system coupled to thefan shaft; and a non-rotatable flexible coupling and a rotatableflexible coupling supporting the epicyclic gear system, thenon-rotatable flexible coupling and the rotatable flexible couplingbeing subject to a Motion I of parallel offset guided end motion withrespect to the engine central axis, the non-rotatable flexible couplingand the rotatable flexible coupling each having a stiffness of a commonstiffness type under a common type of motion with respect to the enginecentral axis, the stiffness being defined with respect to the LFS, thecommon stiffness type is a Stiffness C and the common type of motion isthe Motion I, the Stiffness C of the rotatable flexible coupling beinggreater than the stiffness of the non-rotatable flexible coupling, and aratio of LFS/Stiffness C of the non-rotatable flexible coupling is in arange of 1.5-7.
 11. The gas turbine engine as recited in claim 10,wherein a ratio LFS/Stiffness C of the rotatable flexible coupling is ina range of 16-100.
 12. The gas turbine engine as recited in claim 10,wherein the common type of motion is selected from Motion I, Motion III,or Motion IV, where Motion I is parallel offset guided end motion,Motion III is angular misalignment no offset motion, and Motion IV isaxial motion, and wherein the epicyclic gear system includes a sun gearin meshed engagement with multiple intermediate gears that are rotatablymounted on bearings in a rotatable carrier, each intermediate gear is inmeshed engagement with a non-rotatable ring gear, the sun gear isrotatably coupled to the fan shaft, and the first, non-rotatableflexible coupling is coupled with the non-rotatable ring gear.
 13. Thegas turbine engine as recited in claim 12, wherein the common stiffnesstype is Stiffness A under Motion IV, and a ratio of LFS/Stiffness A ofthe non-rotatable flexible coupling is in a range of 6-25, and a ratioof LFS/Stiffness A of the rotatable flexible coupling is in a range of28-200.
 14. The gas turbine engine as recited in claim 12, wherein thecommon stiffness type is Stiffness D under Motion I, and a ratio ofLFS/Stiffness D of the non-rotatable flexible coupling is in a range of0.25-0.5, and a ratio LFS/Stiffness D of the rotatable flexible couplingis in a range of 2-100.
 15. The gas turbine engine as recited in claim12, wherein the common stiffness type is Stiffness E under Motion III,and a ratio of LFS/Stiffness E of the non-rotatable flexible coupling isin a range of 6-40, and a ratio LFS/Stiffness E of the rotatableflexible coupling is in a range of 4-500.
 16. A gas turbine enginecomprising: a fan; a fan shaft coupled with the fan and arranged alongan engine central axis; a frame supporting the fan shaft, the framedefining a lateral frame stiffness (LFS); an epicyclic gear systemcoupled to the fan shaft; and a non-rotatable flexible coupling and arotatable flexible coupling supporting the epicyclic gear system, thenon-rotatable flexible coupling and the rotatable flexible couplingbeing subject to a Motion III of angular misalignment no offset motionwith respect to the engine central axis, the non-rotatable flexiblecoupling and the rotatable flexible coupling each having a stiffness ofa common stiffness type under a common type of motion with respect tothe engine central axis, the stiffness being defined with respect to theLFS, the common stiffness type is a Stiffness E and the common type ofmotion is the Motion III, the Stiffness E being defined with respect tothe LFS, the Stiffness E of the rotatable flexible coupling beinggreater than the stiffness of the non-rotatable flexible coupling, and aratio of LFS/Stiffness E of the non-rotatable flexible coupling is in arange of 6-40.
 17. The gas turbine engine as recited in claim 16,wherein a ratio of LFS/Stiffness E of the rotatable flexible coupling isin a range of 4-500.
 18. The gas turbine engine as recited in claim 16,wherein the common type of motion is selected from Motion I, Motion III,or Motion IV, where Motion I is parallel offset guided end motion,Motion III is angular misalignment no offset motion, and Motion IV isaxial motion, and wherein the epicyclic gear system includes a sun gearin meshed engagement with multiple intermediate gears that are rotatablymounted on bearings in a rotatable carrier, each intermediate gear is inmeshed engagement with a non-rotatable ring gear, the sun gear isrotatably coupled to the fan shaft, and the first, non-rotatableflexible coupling is coupled with the non-rotatable ring gear.
 19. Thegas turbine engine as recited in claim 18, wherein the common stiffnesstype is Stiffness A under Motion IV, and a ratio of LFS/Stiffness A ofthe non-rotatable flexible coupling is in a range of 6-25, and a ratioof LFS/Stiffness A of the rotatable flexible coupling is in a range of28-200.